1. Field of the Invention
This invention relates, generally, to electrically and thermally conductive adhesives and, more particularly, to adhesives containing a mixture of gallium (or gallium eutectic) and other conductive materials dispersed in a resin.
2. Description of the Prior Art
There are many requirements for electrically and thermally conductive adhesives. One of the specific areas having this requirement is the semiconductor industry. This requirement is especially keen in the process of bonding a semiconductor die (or dice) to a support medium. However, a major problem area exists in the die bonding processes presently utilized by the semiconductor industry. The primary source of this problem is associated with the required use of high temperature (in the range of 175.degree. C. or higher) to secure the die to the device while curing the adhesive.
Presently known materials used for this purpose include powdered gold, silver, copper and metal oxides mixed with a resin. These materials generally result in a volume resistivity of approximately 0.05 to 0.001 ohm-cm and a thermal conductivity of 7 to 12 BTU/(hr. - Sq. Ft./.degree.F/in.). However, use of these conductive adhesives requires approximately 175.degree. C. for curing such that the die adheres to the semiconductor device. This temperature range is likely to contribute to or even cause the failure of many such devices.
Another known process is eutectic bonding. However, eutectic bonding generally requires a temperature of approximately 200.degree. C. (or more) for fusing the die to the device. Such a high temperature is undesirable in many processes in that the die is adversely affected by high temperatures.
Pertinent prior art discovered by applicant relative to this invention is listed herewith as well as in the Prior Art Statement filled herewith. The patent to Prosen (U.S. Pat. No. 3,134,671) discloses a metal alloy with low percentages of gallium. The use is directed to dental appliances and articles of jewelry. The gallium is utilized for lowering the melting point of the alloys.
The patent to Saffir (U.S. Pat. No. 3,513,123) also discloses use of small amounts of gallium in dental alloys for the purpose of modifying the color and for the expansion characteristics of the dental alloys during hardening. The patent to Devaney et al. (U.S. Pat. No. 2,947,646) describes a cellulose acetate film containing colloidally dispersed gallium for use in plastic materials.
Reference is also made to the following publications which are incorporated herein by reference. "Detailed Techniques for Preparing and Using Hard Gallium Alloys", Tech. Note 140, U.S. Dept. of Commerce, NBS, PB - 161641, Harman; "Alloys of Gallium with Powdered Metals as Possible Replacement for Dental Amalgram", Smith et al., Journal of American Dental Association (JADA), Vol. 53, Sept. 1956, pp 315-324; and "Some Physical Properties of Gallium - Copper - Tin Alloys", Smith et al., JADA, Vol. 53, Dec. 1956, pp. 677-685.
Thus, there is a need for adhesive materials having high thermal and electrical conductivity and which do not require high temperatures for curing, this improving the yield in th bonding of devices.